I think MMOs should transcend the client in many ways.

Does anybody remember EQIM? The EQ chat client that logged you into EQ, and let you chat to your friends through tells, but in a sleek IM-style interface? It disappeared after it stopped getting support, but it was a step in the right direction.

I'd like to be able to check my auctions from a webpage. Or send in-game mail. Or trade items. I know that some of this is met with resistance for reasons of exploitation or need to control game traffic or whatever, but I think the MMO that will truly hook me will be the one that I can interact with outside of the client. I'm already addicted to the internet and my iPhone, if I could play, even in a limited fashion, my MMOs on the go, that would be the end of me.

I also like the idea of crafting things offline and loading them into the game, but that's another discussion altogether.